Its all subjective, but "Red Brick" generally means a particular tier of unis. Theres the formal definition. But people use it like that quite alot.
For example when people think of how good or presitigous a uni is. they will generally think.
1) Oxbridge
2) Top london colleges, such as , LSE, UCL, IC,
2) On par but a bit different, top "red bricks" such as Bristol, Sheffield etc
2) On par but slightly different again. Durham, Edinburgh, and the best of the plate glass unis. Such as Warwick, Southampton, etc
3) Lower ranked red bricks such as manchester, liverpool, leeds. and also unis like Exeter, Reading, Sussex.
4) The rest of the plate glass unis, Like UEA
5) Good polies, like Oxford Brookes
6) Worse polies like .. Liverpool John Moores
7) Places that are considered pretty dreadfull such as TVU and Luton.
Sorry if it sounds snobbish, but thats my personal opinion. Its all subjecive and unis will be in different tiers depending on who you talk to. their bias and also what disciplines you are interested in. but generally the structure is as above.